1. Field of the Invention
The present invention relates to a transmission system, a power supplying apparatus, a power receiving apparatus, and a transmission method.
2. Description of the Related Art
Technologies using a magnetic field or radio wave have been developed as technologies to transmit power in a non-contact manner. According to such power transmission technologies using a magnetic field or radio wave, power can be transmitted from a power supplying apparatus to a power receiving apparatus without needing an electric line or the like. Japanese Patent Application Laid-Open No. 2008-21176, for example, discloses an information display apparatus capable of receiving power transmitted by using electromagnetic waves and applicable to a non-contact IC card.
A technique to transmit power using a magnetic field or radio wave will be described below using FIG. 1 and FIG. 2.
FIG. 1 shows an example of the configuration of a transmission system 10 to transmit power using a magnetic field. Referring to FIG. 1, the transmission system 10 includes an AC signal generation unit 12, a primary coil 14, a secondary coil 16, and a circuit load 18.
In the transmission system 10, power is transmitted from the primary coil 14 to the secondary coil 16 by generating an electromotive force based on the law of electromagnetic induction of Faraday. In FIG. 1, if an AC signal is provided from the AC signal generation unit 12 to the primary coil 14, a magnetic flux φ changing with time is generated inside the coil. An electromotive force V corresponding to a change with time of the magnetic flux φ can be obtained by letting the magnetic flux pass through the secondary coil 16. The electromotive force V is given by Formula (1).
                    [                  Math          ⁢                                          ⁢          1                ]                                                            V        =                  -                                                    ⅆ                ϕ                                            ⅆ                t                                      ⁢                                                  [            V            ]                                              (        1        )            
The electromotive force V in Formula (1) is supplied to the circuit load 18.
Next, FIG. 2 shows an example of the configuration of a transmission system 20 to transmit power using a radio wave. Referring to 2a of FIG. 2, the transmission system 20 includes a power supplying circuit 22, a power supplying antenna 24, a power receiving antenna 26, and a power receiving circuit 28. The receiving circuit 28 has a resonance circuit 30, a rectifier circuit 32, and a circuit load 34.
In FIG. 2, a radio wave is emitted with radiation power P from the power supplying antenna 24 of the power supplying circuit 22 to the space. An electric field E [V/m] at a point of the power receiving antenna 26 at a distance d [m] from the power supplying antenna 24 is given by the following formula:
                    [                  Math          ⁢                                          ⁢          2                ]                                                            E        =                  k          ⁢                                                    P                            d                        ⁢                                                  [                          V              ⁢                              /                            ⁢              m                        ]                                              (        2        )            
Here, k is a constant determined by characteristics of the power supplying antenna 24 and the power receiving antenna 26. The power receiving antenna 26 and the resonance circuit 30 can be represented by an equivalent circuit shown in 2b of FIG. 2. R in 2b is a radiation resistance of the power receiving antenna 26 and he is an effective height of the power receiving antenna 26. Effective power Pr of the power receiving circuit 28 is given by the following formula:
                    [                  Math          ⁢                                          ⁢          3                ]                                                                      P          r                =                                                            E                2                            ⁢                              h                e                2                                                    4              ⁢                                                          ⁢              R                                ⁢                                          [          W          ]                                    (        3        )            
Here, if the resonance circuit 30 and the radiation resistance R are matched, a voltage V generated in the resonance circuit 30 is given by the following formula:
                    [                  Math          ⁢                                          ⁢          4                ]                                                            V        =                                            E              ·                              h                e                                      2                    ⁢                                          [          V          ]                                    (        4        )            